Q: Why does dry ice make fog in water?

Dry ice is frozen carbon dioxide, CO2.  At room temperature and standard pressure, carbon dioxide is gas.  To create dry ice, gaseous carbon dioxide is pressurized and refrigerated until it turns into a liquid.  The pressure is then reduced, and liquid CO2 molecules with higher temperature and faster movement evaporate into a gas.  As the faster molecules leave, the temperature of the remaining liquid carbon dioxide drops.  Eventually the liquid freezes into snow crystals, which can then be packed into dry ice blocks or pellets.

At -78.5 degrees Celsius (-109.3 degrees Fahrenheit) and Earth atmospheric pressures, dry ice sublimates.  Instead of the solid melting into a liquid, it automatically becomes a gas.  (The opposite of sublimation, by the way, is deposition– the transition of a gas to a solid.)

 

phase diagram.jpg
Phase diagram for carbon dioxide.

 

This makes dry ice ideal for “no mess” cooling.   Think about using regular ice (frozen water) for cooling.  When the ice melts, you’re left with liquid water.  Dry ice simply disappears, leaving no mess to clean up.  Other uses for dry ice include removing warts and cleaning residue from industrial equipment.

Of course, one of the showy uses of dry ice is in fog machines for nightclubs, theater productions, haunted houses, or other venues that benefit from dense fog.  In these fog machines, dry ice is placed in water.  The dry ice sublimates, creating bubbles filled with CO2.  Liquid water molecules evaporate into the bubble.  The water molecules then condense to form fog.  The bubble separates from the dry ice and rises through the water until it bursts at the surface, emitting the fog.

(Note: Popular opinion was that the fog comes from water vapor in the air.  The cold CO2 bubbles come into contact with air, causing water vapor in the air to condense into fog.  However, this was said to be incorrect in the Journal of Chemical Education by the American Chemical Society.)

You can actually make your own simple “fog machine” at home.  Buying dry ice is fairly easily.  Many grocery stores carry it, though sometimes you may be required to be 18 to buy it.  If you do decide to try this out at home, make sure to follow proper safety precautions while handling the dry ice.  Trust me, you do not want to end up with dry ice burn.

Keep calm and science on.

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